Plant Biology
Phytochrome and Light Sensing
Red/far-red photoreceptor — the reversible Pr↔Pfr switch that reads the color of light
Phytochrome is a plant photoreceptor that reads the color of ambient light by toggling between two interconvertible forms — the red-absorbing Pr (peak ~660 nm) and the far-red-absorbing Pfr (peak ~730 nm). Red light flips inactive Pr into active Pfr; far-red light or darkness flips it back. Because chlorophyll drains red but transmits far-red, this switch turns the surrounding red:far-red ratio into a molecular readout of shade, depth, and season, controlling seed germination, de-etiolation, shade avoidance, and flowering time. The chromophore is phytochromobilin, a linear bilin covalently bonded to a cysteine, whose light-driven Z→E isomerization is the primary photochemical event. Red/far-red reversibility was demonstrated by Harry Borthwick and Sterling Hendricks at USDA Beltsville in 1952, and the pigment was measured spectrophotometrically and named "phytochrome" in 1959.
- Pr peak~660 nm (red)
- Pfr peak~730 nm (far-red)
- Chromophorephytochromobilin (a bilin)
- Genes (Arabidopsis)PHYA–PHYE
- Reversibility shownBorthwick & Hendricks 1952
- Sky R:FR ratio~1.1 sun · <0.2 shade
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Why phytochrome matters
- It is the plant's light-quality sensor. Where the human eye reports brightness, phytochrome reports color balance — specifically the ratio of red (~660 nm) to far-red (~730 nm) light. That single measurement tells a plant whether it is buried, exposed, shaded by a neighbor, or standing in open sun, and it does so with one reversible molecular switch rather than a whole sensory organ.
- It decides whether a seed germinates. Many small seeds stay dormant in soil for years and germinate only when a disturbance brings them near the surface and into red-rich light. A seed reading low red:far-red under a canopy "knows" that a competing plant already occupies the space above and waits. This is direct agronomy: weed seed banks and no-till germination flushes track phytochrome status.
- It drives the shade-avoidance syndrome. Crowded plants elongate stems and petioles, angle leaves upward, suppress branching, and flower early — all triggered by falling Pfr as neighbors deplete red light. In dense monoculture this diverts resources from grain and fruit into stem, and breeding shade-tolerant (rather than shade-avoiding) varieties is an active route to higher planting density and yield.
- It flips seedlings from underground to aboveground development. The transition from the pale, spindly, hooked etiolated seedling to the green, compact, leaf-spread photomorphogenic one is a phytochrome (and cryptochrome) decision. Get it wrong — as constitutive-photomorphogenic (cop) or det mutants do — and a seedling greens up in total darkness and dies before reaching light.
- It calibrates seasonal timing. By feeding the circadian clock's day-length measurement, phytochrome helps set flowering, bud dormancy, and, in trees, autumn senescence. Horticulturists exploit this directly: night-break lighting with red light can force or prevent flowering in commercial poinsettias, chrysanthemums, and other photoperiodic crops.
- It inspired optogenetics. Because phytochrome's Pr↔Pfr switch is fast, reversible, and controlled by two different, tissue-penetrating wavelengths, engineered phytochrome modules (PhyB–PIF, and bacterial BphP tools) are now used to switch protein interactions, gene expression, and cell signaling on and off with red and far-red light in animal cells and synthetic biology.
How phytochrome works, step by step
Phytochrome is a soluble homodimer of roughly 125 kDa subunits. Each subunit folds into a light-sensing N-terminal photosensory module — built from PAS, GAF, and PHY domains — and a C-terminal output module containing PAS-repeat and histidine-kinase-related (HKRD) domains that mediate dimerization and downstream signaling. The photochemistry lives in the GAF domain, which cradles the chromophore.
1. Building the chromophore. Inside the plastid, heme oxygenase cleaves heme to the linear tetrapyrrole biliverdin IXα. Phytochromobilin synthase (the HY2 gene product) then reduces biliverdin to 3Z-phytochromobilin (PΦB). The bilin is exported to the cytosol, where the apoprotein autocatalytically attaches it through a thioether bond between the bilin's A-ring and a conserved cysteine in the GAF domain — no separate enzyme required. A plant that cannot make the chromophore (the hy1 and hy2 mutants) is effectively blind to red and far-red light despite having intact phytochrome protein.
2. The Pr ground state. Freshly assembled phytochrome sits in the Pr conformation, absorbing maximally near 660 nm. In an etiolated, dark-grown seedling essentially all phytochrome is Pr — inactive and largely cytosolic.
3. The primary photoreaction. When Pr absorbs a red photon, the chromophore's C15=C16 methine bridge — the bond linking the C and D pyrrole rings — photoisomerizes from the Z to the E configuration within picoseconds. This tiny rotation is the entire trigger; everything downstream is protein amplifying it. The D-ring flip propagates through a series of spectrally distinct intermediates (lumi-R, meta-Ra, meta-Rc) as the protein relaxes.
4. Conformational output — Pfr. The protein rearranges: a "tongue" hairpin in the PHY domain refolds, the N-terminal extension repositions, and the molecule settles into Pfr, which now absorbs maximally near 730 nm. Pfr is the biologically active state. Crucially, a far-red photon absorbed by Pfr runs the isomerization in reverse and returns the molecule to Pr, and in the dark Pfr also slowly reverts thermally to Pr (dark reversion). Because Pr and Pfr have overlapping absorption bands, any real light source drives the population to a photoequilibrium — so the plant reads a graded Pfr fraction, not a binary on/off.
5. Moving to the nucleus. Light-formed Pfr is imported into the nucleus (phyA import depends on the FHY1/FHL shuttle proteins; phyB carries its own light-dependent signal). There Pfr accumulates in subnuclear foci called photobodies, colocalizing with its signaling partners.
6. Silencing the dark program. In the nucleus, Pfr binds the PIFs (Phytochrome-Interacting Factors), a family of bHLH transcription factors that in darkness keep the plant running the etiolation program and suppressing germination. Pfr binding triggers PIF phosphorylation and degradation by the proteasome. Simultaneously, Pfr inactivates the COP1–SPA E3 ubiquitin-ligase complex, which in darkness destroys photomorphogenesis-promoting factors such as the transcription factor HY5. With PIFs gone and HY5 stabilized, the transcriptome flips: chlorophyll and photosynthetic genes switch on, hypocotyl growth stops, the apical hook opens, and cotyledons expand — photomorphogenesis.
7. Reading the ratio. Every physiological response scales with the Pfr level, which is set by the incident red:far-red ratio. High R:FR (open sun) → high Pfr → compact, green, "safe" development. Low R:FR (canopy shade or a crowded neighbor) → low Pfr → elongation, hyponasty, early flowering. Phytochrome thus behaves less like a light switch and more like a light-quality potentiometer wired to the plant's growth program.
Common misconceptions
- "Phytochrome is a green pigment like chlorophyll." Phytochrome is a protein-bound bilin present at tiny concentrations; you cannot see its color in a leaf. Its job is signaling, not light harvesting. Chlorophyll captures energy for photosynthesis; phytochrome captures information about the light field.
- "Pr is active and Pfr is off." It is the reverse. Pr is the inactive, dark-synthesized ground state; Pfr is the biologically active form generated by red light. Red light generally promotes photomorphogenic responses precisely because it makes Pfr.
- "Phytochrome measures how bright it is." Phytochrome is largely a light-quality sensor. Two lights of identical brightness but different red:far-red content produce very different responses. Brightness (fluence rate) matters mainly for the specialized very-low-fluence and high-irradiance responses of phyA; the canonical low-fluence response is about color, and it is reversible.
- "There is just one phytochrome." Arabidopsis has five (PHYA–PHYE) with divided labor. phyA is light-labile, dominates the very-low-fluence and far-red high-irradiance responses, and mediates germination in near-darkness; phyB is light-stable and the primary sensor of the steady red:far-red ratio behind de-etiolation and shade avoidance. phyC, phyD, and phyE play accessory roles.
- "The response is all-or-nothing." Because the Pr and Pfr absorption spectra overlap, a light source establishes a photoequilibrium with some fraction of molecules in each state. Physiology tracks that Pfr fraction continuously, which is why the red:far-red ratio, not the presence of red light per se, is the meaningful signal.
- "Far-red light does nothing." Far-red is an active signal. A far-red pulse reverses a prior red pulse, and canopy-enriched far-red is exactly what triggers shade avoidance. Far-red is also what makes phyA's high-irradiance response so distinctive: under continuous far-red, phyA — not phyB — drives de-etiolation.
Phytochrome vs cryptochrome vs phototropin
Plants run several parallel photoreceptor systems, each tuned to a different waveband and controlling different behaviors. Phytochrome owns red and far-red; the flavoprotein receptors own blue and UV-A.
| Feature | Phytochrome | Cryptochrome | Phototropin |
|---|---|---|---|
| Wavelengths sensed | Red ~660 nm / far-red ~730 nm | Blue / UV-A ~320–500 nm | Blue ~450 nm |
| Chromophore | Phytochromobilin (a linear bilin) | FAD + pterin (MTHF) | Two FMN in LOV domains |
| Photoswitch | Reversible Pr↔Pfr (Z/E isomerization) | Redox / flavin photoreduction | Cysteinyl–flavin adduct (reversible) |
| Genes (Arabidopsis) | PHYA–PHYE | CRY1, CRY2, CRY3 | PHOT1, PHOT2 |
| Main responses | Germination, de-etiolation, shade avoidance, flowering | De-etiolation, photoperiod, clock entrainment, stomata | Phototropism, chloroplast movement, stomatal opening |
| Reads shade? | Yes — via red:far-red ratio | Partly (blue depletion) | No |
phyA vs phyB — two phytochromes, two jobs
The five Arabidopsis phytochromes are not redundant. The clearest contrast is between the two most abundant, phyA and phyB, which differ in stability, the light regimes they sense, and the responses they control.
| Property | phyA | phyB |
|---|---|---|
| Light stability | Light-labile (Pfr rapidly degraded) | Light-stable (persists in light) |
| Abundance | Very high in etiolated tissue | Lower, roughly constant |
| Signature response modes | Very-low-fluence response (VLFR) & far-red high-irradiance response (HIR) | Low-fluence, red/far-red-reversible response (LFR) |
| Triggering light | Extremely dim flashes; continuous far-red | Red light; steady red:far-red ratio |
| Nuclear import | Requires FHY1/FHL shuttle | Intrinsic light-dependent import |
| Germination role | Triggers germination in near-darkness / deep shade | Classic reversible red/far-red germination control |
| Shade avoidance | Antagonizes it (promotes de-etiolation under far-red) | Primary sensor of canopy shade |
| Flowering | Promotes flowering (long-day) | Tends to delay flowering |
The experiment that revealed the switch
- Hamner & Bonner (1938) — the night matters. Working on the short-day plant Xanthium (cocklebur), Karl Hamner and James Bonner showed that flowering is controlled by the length of the uninterrupted dark period, and that a brief flash of light in the middle of the night (a "night break") could abolish the flowering signal. This pointed to a light-sensitive pigment measuring the dark period — the physiological groundwork for phytochrome.
- Borthwick & Hendricks (1952) — red/far-red reversibility. At the USDA Beltsville lab, botanist Harry Borthwick and physicist Sterling Hendricks, with Marion Parker and Eben Toole, used light-requiring lettuce seed ('Grand Rapids'). A pulse of red light promoted germination; an immediately following pulse of far-red cancelled it. Alternating red and far-red flashes could be strung into a long sequence, and only the last pulse decided the outcome — proof of a single, reversible pigment with two photostates. Their action spectra peaked near 660 nm (promotion) and 730 nm (reversal).
- Butler, Norris, Hendricks & Borthwick (1959) — catching the pigment. Warren Butler and colleagues built a dual-wavelength spectrophotometer sensitive enough to detect the pigment in intact dark-grown maize and turnip tissue. Irradiating with red shifted the absorbance one way; far-red shifted it back — the reversible spectral signature predicted from physiology. They named the pigment phytochrome (Greek phyton, plant + chroma, color).
- Chromophore chemistry (1960s–1980s). Degradation and spectroscopic studies identified the chromophore as an open-chain tetrapyrrole (a bilin), later pinned down as phytochromobilin attached through a thioether bond to a cysteine. The Z→E isomerization of the C15=C16 double bond was established as the primary photoreaction, and crystal structures of bacterial and plant phytochrome photosensory modules eventually visualized the D-ring flip and the PHY-domain "tongue" refolding directly.
- Arabidopsis genetics (1980s onward). Long-hypocotyl (hy) mutants, the constitutive-photomorphogenic (cop/det/fus) mutants, and cloning of PHYA–PHYE turned phytochrome from a spectroscopic curiosity into a fully mapped signaling pathway — PIF transcription factors, the COP1–SPA ligase, HY5, nuclear photobodies, and all — establishing the molecular logic of how one reversible pigment reprograms an entire plant.
Frequently asked questions
What is the difference between Pr and Pfr?
Pr and Pfr are the same phytochrome protein carrying the same chromophore but locked in two different photostates. Pr absorbs maximally in the red at about 660 nm and is the form synthesized in darkness; it is generally considered the inactive ground state. When Pr absorbs a red photon, its phytochromobilin chromophore isomerizes at the C15=C16 double bond from the Z to the E configuration, the protein rearranges, and it becomes Pfr, which absorbs maximally in the far-red at about 730 nm. Pfr is the biologically active signaling form: it moves into the nucleus, binds PIF transcription factors, and triggers photomorphogenic responses. The conversion is reversible — a far-red photon flips Pfr back to Pr, and in prolonged darkness Pfr also decays thermally back to Pr (dark reversion). Because their absorption spectra overlap, any given light source drives the population to a photoequilibrium, so what the plant actually reads is the ratio of Pfr to total phytochrome, not an all-or-nothing switch.
How does phytochrome sense shade?
Chlorophyll in leaves absorbs red light strongly but lets far-red light pass through and reflect. Under an open sky the red:far-red ratio is roughly 1.1 to 1.2, but beneath a leaf canopy — or next to a neighboring plant — red is depleted and far-red is enriched, dropping the ratio to 0.1 or lower. Phytochrome converts this ratio into a Pfr level: high red:far-red keeps a large fraction of phytochrome as active Pfr, while a low ratio pushes the equilibrium toward inactive Pr. Falling Pfr releases the brake on PIF transcription factors and on auxin and gibberellin signaling, triggering the shade-avoidance syndrome: rapid stem and petiole elongation, upward leaf angling (hyponasty), reduced branching, and accelerated flowering — a race for the light. phyB is the dominant sensor of this steady-state ratio in the shade-avoidance response.
What is the phytochrome chromophore?
The chromophore is phytochromobilin (PΦB), an open-chain tetrapyrrole — a linear bilin — chemically related to the mammalian bile pigment biliverdin. It is synthesized in the plastid from heme: heme oxygenase opens the porphyrin ring to make biliverdin IXα, then phytochromobilin synthase (the HY2 gene product in Arabidopsis) reduces it to 3Z-phytochromobilin. The chromophore is exported to the cytosol and attaches autocatalytically through a thioether bond between its A-ring ethylidene side chain and a conserved cysteine in the GAF domain of the apoprotein. Light absorption drives a Z-to-E photoisomerization of the C15=C16 methine bridge between the C and D pyrrole rings, and this single bond rotation is the primary photochemical event that converts Pr to Pfr. Cyanobacterial and bacterial phytochromes use related bilins — phycocyanobilin and biliverdin respectively — attached to a cysteine in an N-terminal PAS domain.
How does phytochrome control seed germination?
Many small-seeded plants germinate only in light, and phytochrome is the light detector that reports whether a seed is buried or exposed at the surface. A brief pulse of red light converts phytochrome to active Pfr, which promotes germination by driving synthesis of gibberellin and repressing the germination inhibitor PIF1 (also called PIL5). This response is fully reversible: an immediate follow-up pulse of far-red light converts Pfr back to Pr and cancels germination — the classic red/far-red reversibility Borthwick and Hendricks demonstrated in lettuce seed in 1952, where the final pulse in a train of alternating red and far-red flashes determined the outcome. In Arabidopsis, phyB mediates this low-fluence, reversible response, while phyA mediates a very-low-fluence response that lets even a millisecond flash of almost any light trigger germination in extremely light-sensitive seeds.
What is photomorphogenesis versus etiolation?
Etiolation (skotomorphogenesis) is the developmental program a seedling runs in darkness underground: a long, spindly, colorless hypocotyl, a protective apical hook, and closed, undeveloped cotyledons that lack chlorophyll — all optimized to punch upward through soil while spending minimal resources. Photomorphogenesis is the program triggered when the seedling reaches light: phytochrome (and the blue-light cryptochromes) convert to their active forms, the hypocotyl stops elongating, the hook opens, the cotyledons unfold and green up, and chloroplast and photosynthetic gene expression switches on. Mechanistically, active Pfr enters the nucleus and destabilizes the PIF transcription factors that maintain the dark program, and it inactivates the COP1–SPA ubiquitin-ligase complex that in darkness degrades photomorphogenesis-promoting factors such as HY5. The switch is what makes a bean sprout grown in a dark drawer pale and leggy, and one on a windowsill green and compact.
How does phytochrome affect flowering time?
Phytochrome feeds into the photoperiod pathway that lets plants flower in the right season. Together with the blue-light receptors and the circadian clock, phytochrome helps the plant measure day length by gating the stability of the CONSTANS (CO) protein: CO accumulates only when its expression coincides with light late in a long day, and stable CO then induces FLOWERING LOCUS T (FT), the mobile florigen that travels from leaf to shoot apex to trigger flowering. Different phytochromes push in different directions — in Arabidopsis, a facultative long-day plant, phyA promotes flowering while phyB tends to delay it, so plants grown under low red:far-red (a phyB-inactivating, shade-like signal) often flower early as part of shade avoidance. In short-day plants such as rice, the phytochrome-and-clock system instead ensures flowering is triggered by long nights. So phytochrome does not set the flowering decision alone, but it calibrates the clock's readout of the light environment.
Who discovered phytochrome?
The physiology came first. In the 1930s Karl Hamner and James Bonner showed that flowering in short-day plants is controlled by the length of the dark period. In 1952 a USDA team at Beltsville, Maryland — botanist Harry Borthwick, physicist Sterling Hendricks, and colleagues including Marion Parker and Eben Toole — demonstrated red/far-red reversibility using lettuce-seed germination and constructed action spectra pointing to a single pigment absorbing near 660 nm and 730 nm. In 1959 Warren Butler, Karl Norris, Hendricks, and Borthwick detected and named the pigment 'phytochrome' spectrophotometrically in dark-grown maize and turnip tissue, measuring the reversible absorbance shift directly. The chromophore's tetrapyrrole nature was worked out over the following years, and the underlying photoreceptor genes (PHYA–PHYE) and the PIF and COP1 signaling machinery were dissected genetically in Arabidopsis from the 1980s onward.